1. Field of the Invention
This invention relates to an automatic focusing device for use in camera; or the like.
2. Description of the Related Art
To continue adjusting an objective lens to focus on a moving object to be photographed by the autofocus system while tracking the moving object, the chance of focus error attributable to the motion of the object must be reduced by compensating for the tracking lag. For this purpose, a prediction method can be considered in which the behavior of a plane of the sharp image of the object is approximated to a quadratic function or a linear function to predict the position which the image plane will take in the near future.
In an autofocus device employing this prediction method, however, despite a smooth change of the position of the (sharp) image plane as is usual, the focus detection and the lens adjustment are made to be alternatively repeated. Hence, the lens moves stepwise. For this reason, a discrepancy between the predicted and actually adjusted positions of the image plane is caused to differ by an amount which change's, as the automatic focus adjusting operation is repeated, depending on a point in the step of movement of the lens at which the focus adjusting operation is shifted to a releasing operation.
To eliminate this drawback, when a camera release is actuated at a point during the repetition of the automatic focus adjusting operation to produce a release start signal, the predicted position of the sharp focus is revised be calculating it again from the past focus adjustment result suit to the time of running of the shutter curtain. After the lens has been adjusted in accordance with the result of that calculation, the shift to the releasing operation is allowed to take place. Thereby, the focus error due to the aforesaid discrepancy is prevented from occurring. The use of this method, however, leads to an increase in time from the moment at which the release start signal has been produced to the moment at which the shutter curtain starts to run, that is, the so-called release time lag. Therefore, the possibility of missing good shutter chances will become high.
Further, in the above-described two methods, the lens repeats the start and stop of movement. This is accompanied with such problems that very frequent shocks are given to the camera and the driving of the lens produces noise, by which the feel of camera use is extremely damaged.
Meanwhile, the other method of keeping the in-focus state stable against the motion of the object is disclosed in Japanese Laid-Open Patent Applications Nos. Sho 62-253107 and 63-5316. In these publications, there is disclosed a technique that as the object is moving, when a tracking lag occurs, the adjustment of the lens is corrected in such a way as to reduce that lag, in which while carrying out the driving of the lens, storage of light (=electric charge) on a focus detecting sensor and computation of the corrected position of the lens are performed. However, this prior known technique does not suggest a positive correction such that the position which the image plane of sharp focus will take in the future is predicted, but is based on the idea of curbing the rise of the tracking lag. So, it will be safe to say that the much-desired improvement of the stability of the in-focus state cannot be expected.